feat(frontend): système de plugins — runtime, menus, layouts custom (#43)

Lots F1-F4 : runtime de chargement/registre plugin, extension des menus
existants, panneau de gestion des plugins, types de layout custom
(sélecteur, fallback, cellule dédiée) branchés sur le port plugin.
Suite npm typecheck/test verte (947/947).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/plugins/runtime/index.ts

@@ -0,0 +1,21 @@
+export {
+  PluginCommandRegistry,
+  PluginLayoutRegistry,
+  PluginMenuRegistry,
+  PluginRuntimeRegistry,
+  type Disposable,
+  type LoadedPlugin,
+  type PluginCommandHandler,
+  type PluginGatewaySet,
+  type PluginLayoutDefinition,
+  type PluginLayoutProps,
+} from "./registry";
+export {
+  loadPlugins,
+  type IdeaPluginContext,
+  type IdeaPluginModule,
+  type PluginActivation,
+  type PluginLoadFailure,
+  type PluginLoadResult,
+} from "./loader";
+export { evaluateWhen, type WhenContext, type WhenEvalResult, type WhenVariable } from "./when";